1. Field of the Invention
The present invention relates to a road-surface friction coefficient estimating device and a road-surface friction coefficient estimating method, and particularly to a method of estimating a road-surface friction coefficient by using an adaptive control theory.
2. Description of the Related Art
A road-surface friction coefficient is used for calculation of control parameters in vehicle control such as traction control, brake force control, torque distribution control or the like. In order to surely perform such control, it is necessary to precisely detect the road-surface friction coefficient. For example, a JP-A-08-2274 and a JP-A-11-64128 that have been already filed by the applicant of the present application disclose technology estimating a road-surface friction coefficient from a steering angle, a vehicle speed and a yaw rate by using an adaptive control theory. According to the JP-A-08-2274, a yaw motion or slide motion of a vehicle is modeled, a yaw motion of an actual vehicle and a modeled yaw motion are compared with each other, and a tire characteristic is momentarily estimated, thereby estimating a road-surface friction coefficient. Furthermore, according to the JP-A-11-64128, a road-surface friction coefficient serving as a base for estimation of a road-surface friction coefficient is set to a predetermined initial value in accordance with a travel environment or a state of a vehicle, whereby the road-surface friction coefficient can be estimated with high precision.
Furthermore, for example, a JP-A-2002-19489 discloses a vehicle-speed controlling device that sets the upper limit value of a vehicle speed smaller as a road-surface friction coefficient decreases, in accordance with the road-surface friction coefficient transmitted from an infrastructure set on a road.
However, with respect to the JP-A-11-64128, the predetermined initial value is properly used according to the situation, however, this initial value may not coincide with the present road-surface friction coefficient. In order to enhance the estimation precision of the road-surface friction coefficient, it is preferable to use an initial value closer to the present road-surface condition, that is, an initial value having higher reliability. Furthermore, the JP-A-2002-19489 discloses control of the vehicle speed by using a road-surface friction coefficient transmitted from an infrastructure, however, it does not disclose any method of autonomously estimating the road-surface friction coefficient by using this value.